The present invention relates to a discharge device for use in an image forming apparatus which discharges a copy item bearing an image fixed thereto to a stacking portion and , particularly to a discharge device capable of discharging envelopes suitably when the envelopes are used as a copy item.
In an image forming apparatus such as a printer and a copying machine, a copy item is discharged to a stacking portion through a discharge guide after a toner image transferred thereto is heated and fused to be fixed thereon by a fixing device including a heating roller and a pressing roller. As a heating roller is used the one in the shape of a one-item hyperboloid since normal sheets are usually used as a copy sheet. The use of this heating roller prevents effectively the production of creases in the normal sheets.
However, there has been the following drawback. The heating roller is in the shape of a hyperboloid. Accordingly, the heating roller has a different rotating speed thereof at a circumferential surface (hereinafter referred to as a circumferential speed) when the copy item passes the fixing roller device. In other words, the closer to the widthwise center of the heating roller, the slower the circumferential speed. When the item passes the fixing roller device, the speed at which the item is transported by the fixing roller device differs in the widthwise direction. As a result, front and rear faces of a portion of the item are transported at different speeds because the pressing roller is cylindrical and the circumferential speed thereof is uniform in the widthwise direction. This will cause the item to warp at this portion. This warp has caused the problems that the item being discharged and stacked on the stacking portion located right downstream thereof comes into contact or collision with the uppermost one of a stack of already discharged items on the stacking portion to disorder or scatter the stack, or causes a jam.
In particular, these problems likely occur in image forming apparatuses recently proposed in which, besides the normal sheets, envelopes are used as copy item and are transported along a single transport path or transported side by side along a plurality of transport paths, and data such as addresses and names are copied or printed on these envelopes. Specifically, in the apparatuses of this type, envelops are set in a cassette for containing the envelopes therein so that turned-up portions of the envelopes are positioned at one side of the cassette with respect to a widthwise direction. The heating roller is in the shape of a hyperboloid, respective side end portions of the envelope in the widthwise direction are positioned at corresponding positions at which the heating roller has a different rotating speed thereof at a circumferential surface when the envelope passes the fixing roller device. Because of the two factors mentioned above, when the envelope passes the fixing roller device, the speed at which the envelope is transported by the fixing roller device differs in the widthwise direction. As a result, front and rear faces of a portion of the envelope including the turned-up portion are transported at different speeds because the pressing roller is cylindrical and the circumferential speed thereof is uniform in the widthwise direction. When this happens, the front and rear faces of the turned-up portion, which are relatively free to move, shift from each other, thereby causing the envelope to warp at this portion.